Aug 20, 2013 9:59 PM

Can anybody identify these large rocks? I found them when digging out a deep soakaway in my garden, about 2.5m deep! My garden being in Faringdon Oxfordshire! The largest measures about 520mm x 420mm and weigh roughly 25kgs, probably more! Would really appreciate an opinion on these! Thanks Steven

I think thay are septarian nodules (a type of concretion), but I'd want to see a cut and polished cross section (or at least cut and wetted) to be sure of that - to see the internal structure properly. I make that ID based on your photos, but also bearing in mind that some old works mention lenticular beds of septaria in the area (a fair match for your specimens), and some more recent research includes concretions, eg.

and which shows some septarian nodules externally and in cross section (to give you an idea what I would be looking for in a polished cross section of yours).

If you were to cut through one of your specimens with an angle grinder using a stone-cutting disc, then wash and wet the surface, you might see something similar (though not as nicely as if sawn on a riock saw then polished). Obviously that would be at your own risk; do take care.

The easiest person/place would be a local funereal stonemason. They are used to dealing with reasonablly-sized pieces of rocks (you might have an architectural stonemason nearby but they'd be less amenable to helping you out because of how they're geared-up to work). If you're lucky you might have another option - a local university earth science department/faculty, where (if you ask nicely) you might be able to find somebody to do it for you.

Think about not just cutting and polishing it to see the inside; think about not so much losing a nodule as gaining a pair of bookends or doorstops...

That is, design your cuts so:

- you get two peices that match (roughly speaking)

- you can use them somewhere they get displayed (if making doorstops, put self-adhesive clear plastic 'bumpers' on the door-facing faces to protect the doors; bumpers like you'd use on cupboard doors) (if making bookends, don't make them too easy to topple)

And aim to keep some of the offcuts (might warrant polishing also) as more-conveniently-sized specimens.

Having designed your cuts, you may find the stonemason has difficulty doing them (because of how the rock needs to be clamped during cutting), so you may have to make some modifications.

Also, since we don't know how strong the rock is, be prepared for some breakage, and give the stonemason guidance on what to do if breakage occurs (eg. "call me!").

Keep us posted!

If you do manage to get it cut and polished (or even cut and varnished), we'd like to see it. We really don't know if it will prove to be boring or spectacular.